Designing a Virtual Reality headset for amblyopic children

Designing a Virtual Reality headset for amblyopic children

Tim R. Elderhorst

This research was conducted on behalf of Vedea Healthware BV in partial fulfillment of the MSc Industrial Design Engineering programme,

Management of Product Development track at the University of Twente.

16-09-2021 DPM 1857

Supervisors

Chair Dr. Ir. D. Lutters

Supervisor Dr. Ir. R.G.J. Damgrave

Supervisor from the company Daniel Jansen

Supervisor from the company Dr. Teun Aalbers

External member Ir. W. Dankers

Acknowledgements

This thesis was written as my graduation thesis for the MSc programme Industrial Design Engineering at the University of Twente. Along with the graduation research, I performed a 9 months internship at Vedea Healthware BV, a medtech start-up focused on developing a no- vel amblyopia treatment for children. During this internship, I was responsible for the design of the VR headset which will be used in Vedea’s endeavours. The process was sometimes difficult and hectic, but mostly it was a valuable and enjoyable learning opportunity, where I learned a lot about VR, but also about product development in general.

I would like to thank Teun Aalbers and Daniel Jansen, two of the founders of Vedea, for giving me this opportunity. I enjoyed our weekly meetings every wednesday morning, and I have learned a lot about how to improve myself professionally.

I would like to thank Roy Damgrave, my mentor at the university, for his help throughout the project. Your insights about VR, the process of product development and writing this thesis have been a great help for me.

I would like to thank Volker Morawe, my colleague at Vedea, for helping me with designing the product. Your enthusiasm and practical tips have helped me a lot during this project.

I would like to thank the team of GainPlay Studio for their collaboration and enthusiasm du- ring this project.

I would like to thank Herman Offerhaus, professor of optics at the University of Twente, for answering my questions about optics. Because I did not have much background in optical science, this part of my thesis has been challenging, but your expertise has helped me with this part of the project.

Lastly, I would like to thank my friends and family for all their support.

To you, dear reader, I hope that you will enjoy reading this thesis.

Tim Elderhorst

Summary

Amblyopia, or lazy eye, is a condition where the visual system suppresses information from the patient’s weaker eye, causing him to lose his stereovision. The condition is most prevalent in Western Europe, where it affects 3,67% of all children. [1] Amblyopia is often detected around the age of 4. It should be treated as soon as possible in order to ensure an effective treatment. If a child is not treated for amblyopia after the age of 8, this can cause lifelong vision problems. [2] [3]

Currently, the most common treatment for amblyopia is occlusion therapy, which requi- res the child to wear an eye-patch over his stronger eye. This method however cau- ses the child to become acutely visually handicapped, diminishing the psycho-soci- al wellbeing and quality of life of the children [4] [5]. Additionally, children are prone to tear off their eye-patches, diminishing the effectiveness and efficiency of the treatment.

Dichoptic training is a relatively new treatment for amblyopia, where patients play certain games or watch certain movies where the eyes are forced to work together. Dichoptic trai- ning forces the visual system to use visual information from both of the eyes, which stimu- lates and enhances the user’s stereovision. [6] It has been proven that dichoptic training is more effective in treating amblyopia than the traditional method of occlusion therapy. [7]

Vedea Healthware BV is a Dutch start-up, dedicated to developing dichoptic training for child- ren aged 4-10. [8] Vedea wants to deliver their dichoptic training by means of a VR headset, which allows children to be more immersed in their training than other methods, as it redu- ces stimuli from outside. A problem that Vedea discovered is that traditional VR devices are not suitable for children, mainly due to the large size and weight of these products. It was found that there is a gap in the current market for VR devices for children. Existing VR devi- ces marketed towards children are in reality not different from regular VR devices for adults.

It was found that in order to make a VR device suitable for young children, it should meet the following three criteria: first of all, the fit of the mask should be less wide in order to fit the size of the child’s head. Secondly, the product depth and weight should be low, in order to decrease the stress on the child’s neck. This is because young children have more difficulty withstanding the forces on their necks than adults. Lastly, the ILD (in- ter lens distance) of the VR headset should match the IOD (interocular distance) of the child, which tends to be much lower than the ILD available in regular VR headsets. Failu- re to match the ILD to the child’s IOD can result in headaches, eye-pain, nausea, double vision, unsharp vision and more problems. [9] An additional remark is that children may have more difficulty with finding the appropriate settings of the VR device than adults.

In order to make a VR headset suitable for amblyopes it has to be taken into consideration that am-

blyopes usually have underlying eye conditions. The most common ones are refractive errors such

as myopia or hyperopia. Slight refraction errors can be solved by changing the lens depth. However,

the best option is to let users wear contact lenses or prescriptionglasses inside the VR headset.

Another common condition among amblyopes is strabismus, which poses additio- nal problems for using a VR headset. In order to make VR headsets suitable for stra- bismic users, a corrective prism should be incorporated into the VR headset. The strength of the corrective prism should correlate with the degree of deviation of the eye.

Lastly, for amblyopes it is vital that the ILD, IOD and ICD (inter camera distance) are equal. When there is a difference between the IOD and ILD or between the ICD and ILD, this causes optical distortion. While distortion should always be avoided in VR, it is extra important during dichoptic training, as it is hypothesized to decrease the effectiveness of the treatment. This is because am- blyopes are already prone to discard the images coming from their weaker eye. Additional distor- tions such as warp and double vision will make it even more difficult for the user to make both of his eyes work together, which is hypothesized to decrease the effectivity of the dichoptic training.

A VR device was designed for Vedea, which meets the requirements that were set during

the project. This product was produced by 3D printing, and will be used during Vedea’s play

tests and clinical trial. The Vedea VR device is 21% lighter than competitor products, and has

a better mask fit. Furthermore, the Vedea VR device has a much lower ILD, and is therefore

much more suitable for children with amblyopia. The product can be used in combination with

prescription glasses, and it contains a click-on optical prism for strabismic users. In order to

make the product viable for mass production, a redesign for injection molded parts is required.

Table of Contents

Acknowledgements 3

Summary 4

Table of contents 6

List of abbreviations 9

Influence chart 10 User experience 11

1. Introduction 12

1.1. Design assignment 14

1.2. Stakeholders 14

1.3. Boundaries 15 1.3.1. Deliverables 15

1.4. Approach 15 1.5. Competitor analysis 17

2. Amblyopia 18

2.1. Amblyopia 20

2.2. Causes of amblyopia 21

2.3. Dichoptic Training 21 3. Virtual Reality 24

3.1. Introduction to VR 26 3.2. Interaction 27 3.3. User experience 28 3.3.1. Immersiveness 28 3.3.2. Fidelity 28 3.3.3. Presence 29 3.3.4. Engagement 29

3.3.5. Expectations 29

3.3.6. Relation to each other 29 3.4. VR device 30 3.5. How VR headsets work 31 3.6. Field of View 31

3.7. Tracking 32 3.8. Display 34

3.8.1. Screen size 34

3.8.2. Resolution 34

3.8.3. Refresh rate 35

3.9. Nuisances to user experience 35

3.9.1. VR sickness 35

3.9.2. Double vision 36

3.9.3. Unsharp vision 36

4. Optics 38

4.1. Our visual system 40

4.2. Focus 41

4.3. Implications of accommodation in VR 42

4.4. Lenses 42

4.5. Lens placement 43

4.6. Aberrations 44

4.6.1. Spherical aberration 45

4.6.2. Chromatic aberration 46

4.6.3. Distortion 46

5. Interocular distance 48

6. Additional research 54

6.1. Forces on the head 56

6.1.1. Head movement 58

6.2. Strabismic users 59

6.3. Conclusion 61

7. Product Strategy 62

7.1. Phase 1: Play tests & clinical trial 64

7.1.1. Requirements 65

7.2. Phase 2: The Minimum Viable Product 65

7.3. Phase 3: The long term product 66

7.3.1. Similarities 67

7.3.2. Room for improvement 68

8. Design Requirements 72

8.1. List of requirements 74

8.2. Practical design guidelines 75

9. Conceptual Design 78

9.1. Variable design 80

9.1.1. Factor model 81

9.2. Lens Design 83

9.2.1. Focal length 83

9.2.2. Lens diameter 84

9.2.3. Lens type 84

9.2.4. Lens material 85

9.3. Part design 86

9.3.1. Front part 86

9.3.2. Rear part 87

9.3.3. Mask 87

9.3.4. Lens tube casing 87

9.3.5. Lens tube 87

9.3.6. Tube front cap 87

9.3.7. Slider 88

9.3.8. Clasp 88

9.3.9. Phone holder 88

9.3.10. Prism tube 88

9.4. Product assembly 89

9.5. Use of the product 89

9.5.1. Customization 90

9.5.2. ILD 90

9.5.3. Lens depth 90

10. Concept Development 92

10.1. Plastic part development - Phase 1 94

10.2. Plastic part development - Phase 2 94

10.3. Material choice 94

10.4. Additional parts 95

10.4.1. Straps 95

10.4.2. Face cushion 95

10.4.3. Lenses 96

10.5. Cost analysis 96

11. Evaluation 98

11.1. Reflection on the project assignment 100

11.2. Reflection on design requirements 101

11.3. Comparison with competitor products 102

11.3.1. ILD 102

11.3.2. FOV 103

11.3.3. Product width 103

11.3.4. Product weight 103

11.3.5. Conclusion 103

12. Conclusion 104

12.1. VR for children 105

12.2. VR dichoptic training for amblyopic children 107

13. Discussion and recommendations 108

Reference list 110

External image sources 113

List of abbreviations

VR Virtual Reality

AR Augmented Reality

XR Extended Reality

MR Mixed Reality

IPD Interpupillary distance IOD Interocular distance ILD Inter-lens distance ICD Inter camera distance

FOV Field of view

Di Image distance

Do Object distance

f Focal length

P Lens strength

vx Vertex distance

MVP Minimum viable product CAD Computer Aided Design

3D Three dimensional

DoF Degrees of freedom

ppi Pixels per inch

PPD Pixels per degree

Influence Chart

User Experience Chart

Chapter 1

Introduction

1. Introduction

Every year, 2.45 million children worldwide are born with amblyopia, also known as lazy eye syn- drome. The highest prevalence of amblyopia occurs in Western Europe, where it is estimated to affect 3.67% of all children. [1] Amblyopia is a condition where the visual system suppresses the visual information received by one of the eyes, which we call the lazy eye. Because this eye is not used, it will become underdeveloped, causing the patient to lose his stereoscopic vision.

Amblyopia needs to be treated at a young age, preferably between the ages of 4 and 7. When children are older than 7 years old it becomes increasingly difficult to treat amblyopia. [2] [3]

The current treatment is called the occlusion method, which requires children to wear an eye- patch on their strong eye for several hours per day. The drawback of this method is that the child acutely becomes severely visually impaired. This causes serious repercussions for the children, ranging from bullying to children not wanting to play outside anymore. It has been shown in multiple studies that traditional methods such as eye-patching and atropine drops diminish the psycho-social wellbeing and quality of life of amblyopic children [4] [5]. Moreover, children who are being treated by occlusion therapy (eye-patching) sometimes tear off their eye-patches, which causes the treatment to be less effective and last longer than intended.

In the last few years, a new treatment method called dichoptic training was developed, which is more child-friendly than traditional treatment methods. Dichoptic training is a treat- ment method where the patient’s stereovision is trained by forcing the eyes to work together.

[6] The underlying principle of dichoptic training is that a different image is presented to the left and right eye. Because the user is presented with two distinct images, the user needs to use both eyes in order to interpret the visual stimuli. This can be done by playing cer- tain games or watching certain videos where two distinct images are provided to both eyes.

Research has shown that doing dichoptic training for 30 minutes per day has a more posi- tive effect on treating amblyopia than wearing an eye-patch for several hours per day [7].

Vedea Healthware BV is a Dutch medtech startup, dedicated to bringing dichoptic trai- ning for children to the market. [8] The company wants to do so by developing a plat- form, containing several dichoptic games and videos for children. The dichoptic training is supposed to be performed using a virtual reality (VR) headset. In order to provide the ser- vice as proposed by Vedea, a VR headset is needed. It was decided as a strategic deci- sion by Vedea that a custom VR headset should be designed and developed for this spe- cific situation. A custom headset can be tailored to the specific needs of the target group.

Furthermore, a custom headset is favourable for the company, as it is cheaper in the long

run and decreases the company’s dependence on third party products. The goal of this re-

search is to find out how a VR headset can be designed to fit amblyopic children aged 4-7.

1.1. Design assignment

The design assignment formulated in this thesis is to design a VR headset suitable for the use by 4-7 year old kids with amblyopia. For this assignment, Vedea is looking for a product that meets at least the following requirements:

• Children should be able to use the smartphone devices of one of their parents.

• Children should be able to comfortably wear the head mounted display for 30-60 minutes per day.

• The head mounted display should be customizable to head circumference.

1.2. Stakeholders

In order to set the right requirements for the product, it is important to define the most important stakeholders and their needs and desires.

The first and foremost stakeholder is Vedea Healthware BV. This company wants to develop dichoptic training for amblyopia patients using VR games. Vedea also gave the assignment to develop and produce the VR headset mentioned in this thesis.

The second stakeholder is Reddito BV. This company is the main stakeholder in Vedea Health- ware BV, and was founded by Daniël Jansen. Daniël is in charge of the business development, marketing and sales of Vedea Healthware BV.

The third stakeholder is GainPlay Studio. This company is one of the three stakeholders in Ve- dea Healthware BV, and is responsible for the research and game development. The company was founded in 2014 by Teun Aalbers and Jan Jonk, and specializes in serious gaming.

The fourth stakeholder is Legio BV. This company is responsible for the platform and software development. The company was founded by Joel Wijngaarde.

Vedea Healthware BV and Vedea’s stakeholders are interested in developing a treatment for

amblyopic children. Vedea is trying to achieve this goal by offering dichoptic training on a VR

headset. The VR headset in question will be a simple VR device, relying on the user’s own smart-

phone. This type of VR device does not contain electronics, a custom display or an embedded

system, which makes the device much cheaper and easier to produce than high-end VR devi-

ces like the Oculus Rift [10]. The added value of Vedea is a new treatment option for amblyopic

children which is more effective and child-friendly than existing methods. Vedea wants to make

this new method available to as many children as possible by starting in the Netherlands and

spreading through Europe. In the process, Vedea is able to annually treat thousands of child-

ren in the Netherlands alone while growing the company. Financial resources will be obtained

through a monthly subscription, which will be paid by the parents of the amblyopic children

or their health insurance company. Vedea concerns itself with developing a software platform

for VR games, the content of these VR games and a VR headset. The platform will be in the

form of a smartphone application, in which the several VR games can be selected and played.

While the knowledge about developing a platform and the VR games is present in Vedea through its stakeholders Legio BV and GainPlay Studio, the same thing cannot be said about the development of the VR device. As Vedea has not yet brought any products to the market, Vedea does not have a regular cash flow and only limited financial resources. This position should be improved by bringing a first product to the market. Vedea will start this process by designing a minimum viable product (MVP) in terms of both platform, VR games and VR de- vice. This MVP will function as a proof of concept for further investments, which should help Vedea to improve their financial position, allowing them to design better products and adding more value to their customers.

1.3. Boundaries

The focus of this thesis lies on the design process of the VR device which will be used for the MVP. The design of Vedea’s platform and content library is also a part of the MVP, but will not be discussed in this thesis. In chapter 7 the company strategy will be explained, making use of a model consisting of three phases. The main focus of this thesis will be on phase 2, which is the design and development of the MVP. Phase 1 consists of the design and development of a prototype, which will be used during the playtests and clinical trial. While this process happe- ned simultaneously with the design of the MVP, it will not be thoroughly explained in this thesis.

Phase 3 concerns itself with the design of an improved product which should be brought to the market several years after the MVP. Although the design of this product will not happen until several years after the end of this thesis, some pointers will be given in this report about which features can be improved in the new product.

1.3.1. Deliverables

The following deliverables are expected to be completed by the end of this project:

• CAD models of all plastic parts

• A plan for the development or purchase of all additional parts (straps, cushion, lenses and closure)

• A prototype of the MVP for phase 1

• A detailed explanation of the variables that influence the product design

• A plan for the production of all plastic parts, including material and production method

• A plan for the use of the product

• A cost analysis of the complete product

1.4. Approach

During this project, a variation on the engineering design process will be used. The design method for this assignment will consist of the following aspects:

Research -> Design requirements -> Conceptual design -> Detailed design ->

Prototyping/Production -> Testing

1. Research

The first phase consists of doing research into various subjects. In order to make a successful product, research should be done about the target group, optical science, amblyopia, the visual system, and many more subjects. The output from the research phase will be used as input for the design requirements.

2. Design requirements

A list of design requirements will be made based on the previous research, which gives the designer a set of constraints during the next phase. The conceptual designs made in the next phase have to satisfy the requirements as indicated in this phase in order to result in a satisfac- tory product.

3. Conceptual design

In this phase the designer will go through a creative process which is called ‘ideation’ or ‘con- ceptual design’. In this phase several concepts will be designed for a given problem. The best concept will be chosen, for which a satisfactory rationale will be given, and this concept will be worked out in detail.

4. Detailed design

In this phase the concept from the previous phase will be worked out in detail. This phase may include the use of calculations, or the use of 3D CAD (Computer Aided Design) modelling. The result of this phase is a detailed part/product design which is ready for production.

5. Prototyping/Production

In this phase the detailed part/product design will be produced. In early instances of this phase a prototype will be produced. If the prototype is satisfactory, this prototype will be tested.

6. Testing

The prototype made in the previous stage will be tested to see if it meets the requirements as stated in phase 2. If the prototype meets the requirements it is deemed a successful prototype.

The design method as seen above is not a linear process. Instead it is a cyclical process, where

the product designer will go through every phase multiple times. Every time the result of a cer-

tain phase is unsatisfactory, the phase will be repeated or the designer will return to a previous

phase. Every problem encountered during the assignment, both big and small problems, will

trigger this design process. Additionally, multiple instances of this design process can occur

simultaneously for different problems.

1.5. Competitor analysis

In the initial stages of this project it was found that some VR headsets for children already exist.

However, upon further inspection it was found that these products in question are not much dif- ferent from regular VR headsets for adults. The examined VR devices in question are the Destek [11] and the Heromask [12]. These devices are marketed towards children of 5-15 and 5-12 years old respectively. In chapter 11.3 will be explained why these products are not suitable for children of these age brackets.

During play tests by Vedea with children aged 4-7 it was found that the Destek and Heromask are too heavy, and the mask fit is too large for children of this age bracket. Furthermore, upon further inspection of these products it was found that the minimum inter-lens distance (ILD) of these products are 57 and 62 respectively. For children aged 4-7, these values are unsuitable in 99% and 100% of the cases respectively. In chapter 5 will be explained why ILD is important for this project.

To conclude, there is a gap in the market for VR headsets for children, and current VR devices

aimed at children do not really fill this gap. In order to make the dichoptic training by Vedea

available to young children (specifically children aged 4-7, but also older children), it is important

that a VR headset for children will be developed.

Chapter 2

Amblyopia

2. Amblyopia

The analysis phase consists of research about several topics that are relevant to this assign- ment. Chapters 2-5 consist of research on various topics that are relevant for this design assig- nment. In order to design a product which will be used to treat amblyopia, it is necessary to first understand what amblyopia is (2.1), and how it is caused (2.2). Secondly, it will be discussed what dichoptic training is and how it can be used to treat amblyopia (2.3).

Figure 1 : girl with amblyopia

2.1. Amblyopia

Amblyopia, also known as “lazy eye syndrome”, is a condition in which the visual system is prioritizing the input from one eye, while discarding the information provided by the other eye. It develops when the brain and eye have trouble working together due to an underlying eye condi- tion. As a result, the brain cannot fully understand the sight obtained from that eye, and chooses to ignore the visual input provided by that particular eye. Over time, the brain will rely more on the other eye, also called the stronger eye. This will cause the weaker eye, or lazy eye, to become underdeveloped. It is estimated that the worldwide prevalence of amblyopia is between 1 and 2%. [13] In European countries, the prevalence is estimated at 3 - 4 %. [1] Amblyopia is also the most common cause of vision loss for kids. The condition usually becomes apparent after the age of 4 [14]. When treated at an early age, kids will regain vision in their weak eye, although it will likely not restore the eye to its optimal state [14]. The older the child gets, the harder it be- comes to treat amblyopia. When amblyopia goes untreated, children may develop lifelong vision problems. [3]

A common symptom of amblyopia is having difficulty perceiving depth. Children with amblyopia also tend to squeeze their eyes a lot, tilt their head, or close one eye when focusing on an object.

In many cases amblyopia goes unnoticed, unless the child is diagnosed by a doctor. Therefore, it is important that all children get a vision screening at least once between the age of 3-5 [3].

There are two existing treatments for amblyopia. The first treatment is using a stick-on eye-patch on the strong eye. This will force the children to use their weak eye, which will strengthen the bond between the weak eye and the brain. Some children need to wear the eye-patch for 2 hours a day, while other children need to wear it every waking moment. The second treatment consists of putting eye-drops of the drug atropine in the strong eye, on a daily basis. The eye-drops will blur the vision in the strong eye, forcing the child to use his/her weak eye. The disadvantages of the current methods are that they greatly reduce the children’s vision during their everyday life.

This has a lot of negative influence on the child’s daily life, and it can cause them psychological

harm. When amblyopic children wear an eye-patch over their strong eye or when they have re-

ceived atropine, they become severely visually impaired. This causes them to avoid playing out-

side and it can also lead to bullying. Additionally, when using the first treatment option, children

tend to tear off the eye-patches. This reduces the effectiveness of the method [4] [5].

2.2. Causes of amblyopia

Amblyopia develops due to an abnormal visual experience during early life, which changes the pathway between the retina of the eye and the brain. This causes the weaker eye to receive fewer visual signals that can be transported to the brain. Over time, the ability of the eyes to work together decreases, which causes the brain to suppress or ignore the information provi- ded by the weaker eye. [3]

Anything that blurs a child’s vision or causes the eye to cross can cause the child to develop amblyopia. [14] The most frequent causes of amblyopia are the following:

• Difference in sharpness of vision between the eyes (refraction amblyopia due to anisome- tropia) is one of the most common causes of amblyopia. A significant difference in refraction of the eye, caused by myopia (near-sightedness), hyperopia (far-sightedness), or astig- matism (a problem in focusing due to irregularities in the cornea of the eye, also called a cylinder deviation), can cause amblyopia. Glasses and contact lenses are typically used to treat refractive issues.

• Muscle imbalance (strabismus amblyopia). The other most common cause of amblyopia is strabismus, a condition where an imbalance in the eye muscles appears in one or both of the eyes. This causes the eye to cross, resulting in diplopia (double vision), hindering the child’s ability to focus on a certain point with both eyes. Strabismus amblyopia can cause one of the eyes to become amblyopic. An optical prism can be used to refract the light in order to temporarily treat strabismus. [15] [16] In order to permanently treat strabismus, eye-surgery is often used.

• Deprivation. A problem in one of the eyes, such as a cloudy area in the lens (cataract) can blur the vision in the eye. It requires early treatment to prevent permanent loss of vision. It is often the most severe type of amblyopia.

The prevalence of different causes of amblyopia differs per study and per ethnic group. [17-20]

According to a study on Australian children, strabismus amblyopia plays a role in more than 50% of all amblyopia cases. [17] A study on Singaporean Chinese children reports that only 15% of amblyopia cases also suffer from strabismus. [20]

2.3. Dichoptic Training

According to a study on dichoptic training for adults with amblyopia [6], dichoptic training is a promising treatment approach for amblyopia. Dichoptic training provides a simultaneous and separate stimulation of both eyes. The image displayed on the strong eye is decreased in contrast, therefore putting the eyesight of the strong eye at the same levels as that of the wea- ker one. Through the use of video games, the weaker eye will start to develop. The effect of dichoptic training on amblyopia patients has been researched in numerous studies. [7] [21-23]

According to Kelly et al. [7], the use of dichoptic training for children proved to be more effec-

tive than the existing eye-patch option. Binocular games that rebalance contrast are mentioned

as a promising additional feature.

The traditional treatments for amblyopia, being patching and atropine drops, focus solely on reducing the visual acuity of the strong eye. Reducing the visual acuity of the strong eye forces the visual system to rely on the weaker amblyopic eye, therefore strengthening the connection between the brain and the amblyopic eye. What these treatments do not offer is training the two eyes to work together. Patching therapy does not address the lack of binocular vision de- velopment, and might even weaken the strong eye [24].

This is where dichoptic training appears to be the superior option, because it trains the two eyes to work together; something that is not trained with traditional amblyopia treatments such as patching or atropine drops. Another big advantage of dichoptic training over traditional me- thods is that it is a more child-friendly approach. Patching and atropine drops cause the child to be visually impaired for multiple hours a day, diminishing the psycho-social wellbeing and quality of life of the child [4] [5]. Dichoptic training only requires the children to play a game for 30 minutes per day, which is much more fun for the children and does not impair them in their daily life.

Dichoptic training does not necessarily include the use of VR (virtual reality) technology. In 2015, game company Ubisoft developed a dichoptic game for amblyopia patients called “Dig Rush” [25]. This game is supposed to be played on a tablet while wearing 3D glasses, with one blue and one red glass. The background of the game only makes use of black, white and grey tints, while the objects that the player can interact with are blue or red. Due to the 3D glasses, the player is able to only see certain objects with his left eye, and other objects with his right eye. In order to play the game, the player needs both of his eyes to work together, which helps them to develop their binocular vision.

Figure 2: example of a dihcoptic training game

While it is not necessary to perform dichoptic training using a VR device, it does offer some benefits. Because a VR device splits the image into distinct parts for the left and right eye, this makes it easier to manipulate the images that the left and right eye will see, which is neces- sary for dichoptic training. Another advantage of VR devices is that they are very immersive, and will therefore probably be better at keeping the user’s attention on the training, which will help to make the training more effective. VR devices also have their disadvantages, namely that they are heavier than for example 3D glasses. This is especially a problem for children, as they have more trouble withstanding the weight of the VR device than an adult would. A paper on the use of VR in dichoptic training [26] states that VR is a promising medium for dichoptic training, but a comparative research between dichoptic training on VR and conventional media is still required. It also mentions the potential disadvantages of visual disturbances, dizziness and nausea, relating to the use of VR HMDs.

Figure 3: gameplay of Dig Rush, a dichoptic game using 3D coloured glasses

Chapter 3

Virtual Reality

3. Virtual Reality

Virtual Reality (VR) is a technology that will be used by Vedea as the medium for their dichoptic training. The first section will explain what VR is and how it works. Secondly will be explained how users can interact with VR (3.2) and how the user experience is constituted (3.3). It will then be explained how a VR headset works (3.4), and which components it contains (3.5). Lastly, this chapter will explain important aspects of the VR device in detail, being the field of view (3.6), tracking (3.7) and the display (3.8).

Figure 4: kid using a VR device

3.1. Introduction to VR

Virtual Reality (VR) is a technology that allows users to experience and interact with a virtual environment. Through VR, users experience a simulated world which can be similar to or diffe- rent from the real world. When using VR, users are partly cut off from the real world, in order to experience the virtual world as if they were inside of the virtual world. VR can be used for several purposes, such as entertainment, education, medical treatment or in business settings. Usually when using VR, multiple of the user’s senses are being stimulated. These are usually seeing and hearing, as they are the easiest and most valuable senses to simulate. In some cases the sense of touch is simulated through the use of haptic feedback systems such as haptic gloves or treadmills. There are instances where smell and taste are stimulated in VR, although most VR systems do not make use of this.

While other technologies such as non-VR video games and movies also contain simulated re- alities, they are not considered VR. The reason for this is that video games and movies do not replace the user’s presence in the real world with a presence inside a virtual world. Instead, the technologies take up a place within the real world through artifacts such as display screens and loudspeakers.

VR is part of a family called extended reality (XR). [27] XR is a family of technologies whose function is to replace or add to the user’s reality. Another subgroup of the XR family is augmented reality (AR), which is a technology that adds certain experiences to reality, without replacing it with a completely virtual reality. An example of AR is the game “Pokemón Go” [28], where images of digital characters are displayed on the user’s phone screen, as if they were present in the real world. A third subgroup of XR is mixed reality (MR), which is a combination of both VR and AR.

The way in which VR manifests itself does not always have to be the same. In most cases,

the user uses a head-mounted display (HMD), also known as VR Goggles or a VR device. A

head-mounted display is a sort of helmet containing a display screen. The content of the virtual

world is displayed to the user through the display screen of the HMD. There can however also be

VR systems that do not make use of HMDs, but instead use one or more large display screens

in order to display the virtual world. Additionally, VR systems sometimes use sound or touch in

order to stimulate the user beyond the optical part, although this is not a requirement for VR. The

use of touch can for example be implemented in different ways, such as through haptic gloves

or treadmills.

3.2. Interaction

In many VR systems, users can interact with the virtual world in a certain way. One of the most common means of interaction is one where the user can look around in the virtual world as if he was really there. This is done by means of a gyroscope, which is a device that tracks rotation.

The use of a gyroscope allows the user to look around in the virtual world by means of rolling, pitching and yawing. A VR system that allows these three elements is called a 3 DoF (degrees of freedom) system. In some VR systems, the user can not only interact with the virtual world by rotating, but also by means of moving in a certain direction, which involves the use of a posi- tion tracker. Using a position tracker allows the user to move in certain directions, by means of strafing, elevating and surging. A VR system that allows both rotation and movement is called a 6 DoF system.

There are other ways in which users can interact with a VR system. Some VR systems make use of controllers. In this case users can for example use a joystick to move around, and press buttons to perform certain actions. Other VR systems make use of a haptic feedback glove:

a glove that tracks the movement of your hand and fingers, allows you to interact with the VR world by e.g. picking up objects, and provides the user with physical resistance when touching a virtual object. Some VR systems use a treadmill, which allows the user to walk around in the virtual world by actually walking in different directions. Interaction systems such as the tread- mill and the haptic feedback glove give users a sensation that is very similar to the real world, while other interaction systems such as a controller offer simpler and less sophisticated ways to interact with the virtual world.

Figure 5: the six degrees of freedom in a VR application

3.3. User experience

While VR can be used for many different purposes, the goal of VR generally is to provide the user with an immersive experience. According to M. Slater, a good VR experience should be immersive, engaging and make the user feel present in the virtual world. [29] The combination of these variables is what is called user experience (UX). This chapter will explain in more detail the variables that contribute to a good user experience.

3.3.1. Immersiveness

Immersiveness is a term that is often misused to describe either presence or UX. What immer- siveness actually describes is how well a VR system mimics the sensory experiences of the real world. Immersiveness relates to the form of the VR experience instead of the content. When watching a movie in a cinema with an IMAX screen and surround-sound, the immersiveness is much higher than when watching the same movie on your smartphone screen in the train. This has nothing to do with the content of the movie itself, nor is it a subjective experience of the user.

Immersiveness is an objective property which can be measured. The immersiveness can e.g. be increased by having a higher field of view (FOV), a higher image resolution, or by having more possibilities for interaction with the VR device.

3.3.2. Fidelity

Where immersiveness refers to the form of the VR experience, fidelity refers to the content. Fide- lity explains the level of detail of the content of the VR experience. This has nothing to do with the hardware product, but relates entirely to the software content, such as the movies or games that make up the VR experience. Similar to immersiveness, fidelity is an objective and measurable property which describes the simulated environment. In a book, fidelity could describe the level of detail in which the world and characters are described. For a movie, fidelity could e.g. refer to how lifelike the animations look. If you compare The Lion King (1994) to The Lion King (2019), you see that the 2019 version clearly has a higher fidelity. Having a higher fidelity however does not mean that the movie is better or more engaging than movies with lower fidelity. The level of fidelity is a choice of the developers, and the goal is not always to maximize fidelity.

Figure 9: comparison between the Lion King 2019 (left) and the Lion King 1994 (right)

3.3.3. Presence

Presence refers to the degree to which users feel present within the virtual environment. Similar to immersiveness, presence describes the form of the VR experience, and not the content itself.

Contrary to immersiveness and fidelity, presence is a subjective experience of the user and not an objective property of the VR experience. Theoretically, having a higher immersiveness and fidelity increases the user’s presence in the virtual environment. However, this is not always the case, as having higher immersiveness and fidelity will also increase the user’s expectations of the VR experience. When these expectations are not met, the user’s presence will be disrupted.

3.3.4. Engagement

Engagement, also known as interest or involvement, explains the user’s cognitive reaction to the content of the VR experience. [29] When a user is very engaged with the content, he enters a state of extreme focus which is called the flow-state. When a user is not engaged, he will become bored and focus on things outside of the VR experience. Note that the level of engagement is not necessarily related to the immersiveness or fidelity. Someone can watch a badly animated movie on his smartphone screen in the train, and still be very engaged with the movie. Similarly, a movie with lower fidelity and immersiveness can be more engaging to a user than another movie with higher fidelity and immersiveness. However, in general having a higher immersiveness, fidelity and presence will increase the user’s engagement. Engagement is a psychological and subjective process, relating to the content of the VR experience. It is there- fore not an objective property of the VR experience.

3.3.5. Expectations

Being subjected to a certain VR experience will cause the user to develop certain expectations from this experience. If these expectations are not met, it will disrupt the user’s presence and engagement with the content. Expectations can be seen as a barrier that connects immersive- ness and fidelity (objective properties of the VR experience) to presence and engagement (psy- chological processes happening to the user). As mentioned before, having a higher fidelity and immersiveness will increase the expectations of the user. However, when these expectations are not met, this will have a negative effect on the user’s presence and engagement.

3.3.6. Relation to each other

The relationship between immersiveness, fidelity, presence, engagement and expectations is complicated. While all of these factors influence each other, they are also in some ways inde- pendent of each other. A movie can have a low fidelity but still be very engaging. Or another movie can be very engaging, although the person watching it may not feel a strong sense of presence. Immersiveness and fidelity relate to objective characteristics of the virtual environ- ment, while presence and engagement refer to the user’s individual psychological state. At the same time, presence and immersiveness refer to the form of the VR experience, while engage- ment and fidelity refer to the content of the experience.

In order to design a great user experience, the goal should not always be to maximize all five

parameters. Instead, it should be determined which factors are important for this particular

project, and adjust the parameters accordingly. [29] In the case of Vedea’s VR experience, the

fidelity and immersiveness do not need to be high, and neither does the presence. The most

important part of the Vedea method is that the engagement is high. This is however difficult to objectively measure, as engagement is a subjective reaction to other variables. The VR con- tent should be engaging enough that users want to use the product on a daily basis for 30-60 minutes per day. If users are more engaged with the treatment, this means that the treatment will be more effective.

3.4. VR device

VR headsets, also known as VR goggles or head-mounted displays (HMDs), are devices that allow people to enter a virtual world. A VR headset blocks the view of the real world, allowing users to become more immersed in the virtual world. In the virtual world, several contents can be displayed. This can consist of images, video, sound, interactive games or a combination of these aspects.

VR devices are the easiest and most common way to use VR technology. This is because it is relatively cheap and simple when compared to other methods, such as using large display screens all around the user. There are many different types of VR devices, which can vary greatly on price and quality. Take a look at the Google Cardboard [30] for example, which is a device that costs only 10 euros. The product is entirely made of cardboard, except for the lenses and head strap. The Google Cardboard makes use of the user’s smartphone, which will act as the display screen, embedded system and speakers of the VR device. On the other hand there are products like the HTC Vive [31], with a custom screen, embedded system, controllers and high quality parts. Needless to say, the HTC Vive offers a much higher quality, but also at a much higher price (800 euros for the Vive Cosmos). Then there are the professional VR products, such as the Varjo-VR3 [32], which offers a superior image resolution when compared to other VR products, but at a cost of more than 8.000 euros. Later on in this chapter there will be more information about the different specifications of a VR device, and how different VR devices compare to one another. However, first an explanation will be given about how a VR device works.

Figure 10: Google Cardboard (left) and figure 11: HTC Vive Cosmos (right)

3.5. How VR headsets work

The way in which a VR headset works is by splitting the visual input into two distinct parts.

The left eye will look at a slightly different image than the right eye, due to the slightly different camera perspective in which the content is displayed. This small difference in perspective will create a false sense of depth in the VR world, simulating how our eyes would function in the real world. [33] As mentioned before, the VR device will split the image into two parts, while dividing the two halves with a small insert so that the left eye cannot see the right half of the display and vice versa.

Another important aspect of the VR device is the lens. Because the screen of the VR device is situated very close to the user’s eyes, this will put a strain on the user’s eyes and make it impos- sible for the user to focus on the image. [34] [35] In order to reduce the strain on the eyes and allow the user to see the image clearly, a pair of additional convex lenses should be added to the VR device. This will allow the ciliary muscles in the eyeballs to relax, while providing a sharp and magnified image to the user. In order to provide a sharp image, the placement of the lenses is crucial. More explanation about the lenses will be provided in the next chapters.

3.6. Field of View

An important term in VR is the field of view (FOV). FOV is important because it explains the de- gree to which people are able to see the virtual world through the VR device. A higher FOV will better approach the FOV of the real world, which will lead to a more immersive VR experience.

FOV explains the part of our view which is occupied by a certain object, in the case of VR this object is the display screen which displays the virtual world. In the real world, humans have a vi- sual field of 200 to 220 degrees horizontally and 130 to 135 degrees vertically. [36] With regards

Figure 12: content of a VR HMD

Figure 13: the horizontal (left) and vertical (right) human visual field.

In VR it is especially important to achieve a large FOV. When you are watching TV, you are looking at an object situated in the real world. In VR however, a virtual world is being simulated, which replaces your view of the real world. In order to create a realistic VR experience, the FOV of the virtual world must approach the visual field of the real world. In order to achieve this, the FOV should be significantly larger than that of a TV or smartphone for example. As it is not convenient to design a very large display (regarding weight and resources), the best option to achieve a high FOV is to place the display close to the user’s eyes. This also has its drawbacks however, as humans have trouble focusing on nearby objects. [35] This problem can however be solved by using optical tools called convex lenses. More information about convex lenses will be given in chapter 4.4.

3.7. Tracking

In order to make it possible for the user to look around and move in the virtual environment, VR devices make use of tracking. [33] There are two types of tracking, the first being head

to the horizontal visual field, the centermost 120 degrees make up our binocular view, which means that this is the part of the view which can be seen with both eyes. The parts of the view on the sides of the binocular view are called the temporal crescent, which can only be seen with one of the eyes. This is also called the monocular view.

Whenever we look at a certain object, this object takes up a certain part of our view. An object will have a large FOV when its size is large with respect to the distance between the observer and the object. The FOV of an object can be calculated with the following formula:

FOV = 2 tan

(0.5 * width / eye-to-screen distance)

For example, when we look at a TV screen with a display size of 80 cm from a 2 meter distan-

ce, the diagonal FOV of the TV screen will be 22.6 degrees.

Figure 14: FOV of a tablet and phone (left) and FOV of a VR device (right), compared to the human visual field

There are two distinct ways to deliver positional tracking. This can happen either by inside-out tracking or outside-in tracking. Inside-out tracking is when a sensor inside the VR device tracks its surroundings in order to determine the user’s relative position in the room. This can be done by having a camera scan certain markers in the room, or by simply scanning the features of the environment. Inside-out tracking does not necessarily have to be done using a camera. It can also be done using IR (infrared) sensors, or any other type of optical sensor.

Outside-in tracking is characterized by having sensors placed in the environment instead of in the VR device. In outside-in tracking, the position of the user is tracked by tracing the position of the VR device with regards to the sensors. It is common that the VR device contains markers on or in the device, which help the sensors to detect the device more easily. While all optical sensors can be used for outside-in tracking, the most common type are IR sensors.

Figure 15: an example of inside-out and outside-in tracking

tracking (enabling the user to look around, by pitching, yawing and rolling) and the second

being positional tracking (allowing the user to move, by strafing, surging and elevating). A

distinction can be made between 3 DoF (degrees of freedom) and 6 DoF systems. The first

one is the most simple variant, where the device only makes use of head tracking. This can

be done by using a gyroscope. The latter makes use of both head tracking and positional

tracking. This is a more advanced version of tracking which allows the user more opportunity

to move in the virtual environment.

3.8. Display

The display is a vital part of any VR device, because it displays the content which forms the virtual environment for the user. This section will explain how displays can differ in different VR devices. Moreover, this section will explain the concepts of screen size, resolution and refresh rate, and how these variables affect the VR experience.

The display is the part of the VR device that displays the VR content to the user through the lenses. VR devices on the higher end of the spectrum usually contain custom made displays, while lower end devices usually rely on the user’s smartphone as visual input. There are three important properties of the display: screen size, resolution and refresh rate. A high screen size, resolution and refresh rate are desirable qualities for a VR device, but they will also increase the price.

Figure 16: example of an LCD display screen (left) and figure 17: a display screen of an iPad (right)

3.8.1. Screen size

The screen size influences the FOV of the VR device. The larger the screen size, the higher the FOV can be. The FOV can however be limited by the size of the lens or the size of the looking hole. VR devices with a custom display usually have a higher FOV than devices that rely on the user’s smartphone. The reason for this is that the latter devices need to have a small looking hole in order to be compatible with smaller phones.

3.8.2. Resolution

The resolution of the display screen is important in VR, even more than in other electronic de- vices. The goal of VR is to simulate the real world in a virtual environment. In order to do this realistically, the resolution of the virtual environment should approach the resolution of the real world. The highest resolution that the human eye can see is approximately 60 pixels per degree (PPD). When looking at a tv, laptop or smartphone, a 60 PPD resolution can easily be achieved.

This is what is called high definition (HD) quality. In VR it is however much harder to achieve a

high resolution. The main reason for this is that the display is located very close to the user’s

eyes and is subjected to additional magnification by the lenses, which results in a heavily mag-

nified image. When looking at a flat screen, the part right in front of the user’s eye will have a

lower resolution than the parts that are located more towards the edges. The parts towards the

edges will however not be well visible, due to the higher distortion of the lens towards the edges of the lens. A display screen with 400 pixels per inch (ppi) resolution would result in a resolution of approximately 12.5 PPD in front of the lens center when used in VR. This corresponds to a visual acuity of 20.8%. Current displays usually have a resolution of roughly 300 to 600 ppi.

The former corresponds to a foveal vision of roughly 9.4 PPD, while the latter corresponds to roughly 18.8 PPD.

3.8.3. Refresh rate

The refresh rate of the display screen determines how fast the pixels on a screen that form the image are being refreshed. The refresh rate is expressed in Hertz (Hz) which means the num- ber of times per second that the image changes. While refresh rate is important when playing regular video games, it is especially important in VR. Because the purpose of VR is to immer- se the user in a virtual environment, a low refresh rate makes the user feel as if his eyes are not working correctly. Low refresh rates can lead to ‘VR sickness’, a condition that may result in nausea, headaches and disorientation. [41] The general consensus among VR users and designers is that 90 Hz is a good starting point for VR. Anything under 90 Hz can cause the problems mentioned above. [41]

Nowadays, display screens come in many different refresh rates, ranging from 30 to 144 Hz.

Almost all smartphones nowadays have a standard setting of 60 Hz. However, in some cases it is possible to increase this rate to 90 or 120 Hz. If possible, the refresh rate should be set at the highest setting for the optimal VR experience. The downside of a high refresh rate is that it will deplete the battery faster, and it will require more GPU power. If possible, the refresh rate should be at least 90 Hz, as having a lower refresh rate can cause problems to the user.

3.9. Nuisances to user experience

In this section several effects will be explained which can cause nuisances to the user experi- ence. The most common nuisances related to VR technology will be listed below. In this section will be explained how VR sickness, double vision and unsharp vision can be avoided when using a VR device.

3.9.1. VR sickness

Some users of VR devices experience feelings similar to motion sickness when using VR. The- re are several factors that may contribute to this feeling of sickness related to the use of VR.

• Low refresh rate (Hz)

Having a refresh rate below 90 Hz can cause nausea, headaches and disorientation to the user. [37] Refresh rate determines how many times the image input is refreshed. A low refresh rate will be noticeable by the user, and cause the user to notice the difference between the real world and the virtual world.

• The vergence-accommodation conflict.

This conflict is caused by the illusion of depth which is caused by VR. The difference between

the visual input of both of the user’s eyes makes it seem like he is looking at objects at different distances from him, while in fact the user is looking at a screen, projected infinitely far away from him. When a certain object on the screen appears to be closer or farther away than the object the user is currently viewing, the user will instinctively accommodate. While accommo- dation is useful in the real world, in VR it causes users to lose their focus on the screen, as their eyes are no longer focused at infinity. The vergence-accommodation conflict applies to all current VR devices, and may cause nausea or eye-strain. [38]

• Mismatch between image input and vestibular input

Vestibular input coordinates movements of the eye, head and body, which affects our body’s balance, muscle tone, visual-spatial perception, auditory-language perception and emotional security. When a VR user senses a mismatch between the image input and the vestibular in- put, this can create a sense of uneasiness, dizziness, disorientation and nausea. This can for example happen when the user rotates his head, but his perception of the virtual world does not change accordingly [39].

• Mismatch between the IOD and ILD

According to Regan and Price [9], users with an IOD (interocular distance) smaller than the ILD (inter-lens distance) of the VR device will experience a range of problems, such as bino- cular stress, increased near-point convergence, fatigue, eye-pain, blurred vision, headaches and nausea. According to Kolasinski [40], a mismatch between the IOD and ILD is one of the reasons that VR sickness can occur. More information about the IOD and ILD will be given in chapter 5.

3.9.2. Double vision

Double vision is caused when the distance between the lenses (ILD) does not match the in-ga- me distance between the camera’s (ICD). According to Regan and Price [9], double vision can also occur when the user’s IOD (interocular distance) is smaller than the ILD of the lenses. For an optimal experience, the ILD and ICD should be equal to the user’s IOD for non-strabismic users. The IOD is the distance between the centers of the user’s eyes, which is a physical characteristic of each individual user. More information about the IOD, ILD and ICD is given in chapter 5.

3.9.3. Unsharp vision

The primary cause of unsharp vision in VR is when the depth positioning of the lenses is incor- rect. Placing the lens too close to the display will make the virtual image appear more nearby, resulting in eye-strain and in extreme cases unsharp vision. Placing the lens too far away from the display will cause the image to form in front of the retina, resulting in an unsharp vision. In order to achieve sharp vision without eye-strain, the user needs to find his own ‘sweet spot’.

This is the spot where the image forms on the user’s retina, without the user needing to flex the ciliary muscles in the eye.

Another cause of unsharp images in VR is spherical aberration of the lenses. Spherical aber-

ration causes the light rays to bend differently when they enter the lens further away from the

optical center of the lens. Due to spherical aberration, light rays passing through the edge of

the lens will have a shorter focal length than light rays passing through the center. As a result,

the user may be able to see sharp images through the center of the lens, but not through the

edges of the lens. Spherical aberration can be limited by choosing lenses that limit spherical

aberration for VR applications. This will be further discussed in chapter 4.6. Another way in

which spherical aberration can be limited is by choosing lenses with fewer lens strength. This

however creates other unwanted outcomes, being that the size and weight of the product will

increase, resulting in an increased strain on the user’s neck.

Chapter 4

Optics

4. Optics

Before we can understand how a VR device works, we must first understand the workings of the human visual system. In the first section will be explained how the human visual system works, and how it allows us to see objects (4.1). Afterwards, it will be explained what accom- modation is, how it allows us to focus on certain objects (4.2), and why it is important in VR (4.3). The following sections focus on lenses (4.4), why the placement of the lens is important (4.5) and which optical aberrations occur in lenses (4.6).

Figure 18: schematic drawing of the optic nerve

4.1. Our visual system

Our visual system consists of the eyes, along with some parts of the central nervous system.

The eyes register visual information in the form of light, and this information is processed in the brain by the visual cortex. A neural pathway connects the eyes with the visual cortex, and allows the information to transport from the retina to the brain. [41]

Our eyes are the organs of the visual system that enable us to see visual details. The eyes can see objects when light is reflected by the object into the eye, through the cornea, pupil and eye lens onto the retina. The eye lens converges the incoming light rays, and converges them into a single point called the image point. If the image point is situated at the retina, this results in sharp vision. If the image point is located before or behind the retina, this results in blurred vi- sion. The eye lens has the ability to become more and less convex, thereby changing the focal length of the lens. This process is called accommodation, and is necessary for us to change our focus from near to far away objects. [41] [42]

Accommodation happens unconsciously, therefore people are usually unaware that their len- ses are constantly changing in strength. [41] The closer that an object is located to the eye, the more the lens has to bulge in order to place the image point on the retina. When an object is far away, the ciliary muscles in the eye stretch, increasing the focal length of the lens. When an object is nearby, the ciliary muscles in the eye compress, causing the focal length of the lens to decrease. [41] [42]

The retina is made up of rods and cones, which convert the light into electrical impulses which are sent to the brain. The cones allow us to see colours, and are used especially during the day. The rods are very sensitive to light, and allow us to see light in dark environments. The space on the retina which is placed directly behind the pupil and lens is called the yellow spot.

This spot contains the highest concentration of cones, and is therefore able to see the highest

quality of vision. This type of vision is called foveal vision or central vision, and makes up 1.5-2

degrees of our total vision. [36] The vision which is located outside of the fovea is called perip-

heral vision. This type of vision is significantly less sharp than the foveal vision, due to the low

concentration of rods and cones outside of the yellow spot. Although the resolution of the pe-

ripheral vision is very low, this type of vision is still useful for seeing fast movements or general

shapes and colours, which draw the attention of the fovea.

4.2. Focus

Focusing on a certain object requires more than simply seeing the light rays coming from that object. The only way to see a clear image is to bring the image into focus. This is done by con- verging the light rays coming from a point on an object into a single point, and placing this point on the retina. This point is called a focus point, or image point. Every object is made up of a mul- titude of points, and in order to focus on the object all these points should be converted to image points and placed on the retina. Light rays naturally diverge when coming from an object, and they converge after leaving the lens of the eye. The distance between the eye and the object determines the distance between the eye lens and the image point. When an object is far away from the eye, the resulting image point will be close to the eye lens. When an object is nearby, the resulting image point will be further away from the eye lens. As mentioned earlier, we can only focus on an object when the image points associated with that object are placed exactly on the retina. If the image points are placed in front of or behind the retina, the resulting image will be vague. [41] In the figure below it can be seen that for a certain lens strength, only objects at a specific distance can be put into focus.

Figure 19: the focus of light rays with different object distances.

Luckily, our eyes are able to change the strength of the eye lens, which allows us to focus on ob- jects at different distances. In a relaxed state, our eye lenses are very strong, containing a lens strength of 60 diopters. This strength is needed to converge horizontal light rays into an image point in only several centimeters. When the ciliary muscles contract, the eye lens can gain an additional strength of up to 16 diopters in a process called accommodation [35].

Accommodation happens unconsciously, and allows us to focus on more nearby objects as well as far away objects [35] [41]. Focusing on faraway objects is the least difficult for our eyes, as our eyes can focus on objects infinitely far away when our ciliary muscles are in a relaxed state.

Focusing on nearby objects requires more effort of the ciliary muscles. There is also a limit on

which distances can still be focused on. This limit is called the near point. For a young child, this

near point is situated between 5 and 10 cm away from the eyes. The distance to the near point

however increases over time, because we lose the accommodative power of the eyes. [41] If

a person has a near point 20 cm away from his eye, he will not be able to clearly see objects

that are more nearby than 20 cm. It is possible to see an object at 20 cm or further away from

the eye. It should be noted that looking at nearby objects requires more effort from the ciliary

muscles than looking at faraway objects. [35]